The field of this invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to ablation devices for use during MRI guided thermal therapy.
Thermal energy deposition is often used in medicine as a means of necrosing diseased tissue. Lasers, radio frequency antennas and ultrasonic transducers are examples of devices used for the deposition of thermal energy for therapy. It is desirable to have a means of guiding and monitoring this energy deposition to assure the energy is applied in the proper location and to verify that appropriate energy levels are used to prevent undertreatment or overtreatment. Magnetic resonance imaging has been demonstrated as a method for identifying regions of tissue to be treated, guiding therapeutic- devices and biopsy needles as well as monitoring the deposition of thermal energy from lasers, ultrasound devices or cryogenic probes.
Intra-operative MR imaging is employed during a medical procedure to assist the physician in guiding an instrument. For example, during ablation therapy the MRI system is operated in a real-time mode in which image frames are produced at a high rate so that the location of the ablation device can be monitored as it is inserted. A locator device, such as that described, for example, in Dumoulin et al. U.S. pat. No. 5,271,400 issued Dec. 21, 1993 or Dumoulin et al. U.S. pat. No. 5,307,808, both of which are assigned-to the instant assignee, may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes that is sensed by three stationary cameras.
Systems which employ the MRI system to locate markers in the medical device have also been developed. Such tracking systems employ a small coil attached to a catheter or other medical device to be tracked. An MR pulse sequence is performed using the tracking coil to acquire a signal that indicates location of the tracked device. The location of the tracking coil is determined and is superimposed at the corresponding location in a medical image acquired with the same MRI system.
Other techniques to perform ablation therapy also exist. Some of these techniques cannot be used in the high magnetic field and low noise environment of an MRI system. Any device used in the bore of the magnet can distort the precise magnetic and radio frequency fields required for accurate imaging. In addition, the current carried by electrical devices produces a local magnetic field that produces artifacts in the acquired images.
An ablation system which employs a resistive device is guided into position using MRI and is then energized with electric power to treat target tissues. The system includes an ablation device that contains a resistive element, an MRI system which acquires image data from a patient undergoing treatment with the ablation device, and an ablation control for providing alternating current to the resistive element to produce heat for treating target tissues. The frequency of the current is set to a value such that the alternating current does not produce artifacts in the images reconstructed from the acquired image data.